It has been and is a common practice for drivers of the combines to drive each combine having a pair of right and left crawlers such that it runs along and harvests a row of grain stalks in a farm and then turns itself at an end bare area of the farm before advancing to the next row of grain stalks. The combine has therein a transmission receiving a traction power from an engine and outputting it to the right and left crawlers through a right-side and left-side clutches, respectively. When turning the combine at the bare end, the driver has to switch off one of those clutches to temporarily make idle one of the crawlers. It is disadvantageous that this operation must be done quickly almost at the same time as changing the speed of those crawlers, also undesirably increasing the radius of turn.
It may be possible to employ a pair of oil-hydraulic stepless speed changers to transmit power from the engine to the right and left crawlers, independently of each other. In this case, operations for speed reduction will be easier and the radius of turn will be decreased easily to a noticeable extent. However, it will become more or less difficult for the drivers to steer such a combine to run straight or along somewhat curved rows of grain stalks.
Alternatively, a single oil-hydraulic stepless speed changer may be employed in combination with an oil-hydraulic `steering gear` or steerage. The former speed changer will transmit engine power in unison to both the crawlers, whilst the latter device increases the speed of outside crawler remote from the center of turn, reducing at the same time the speed of inside crawler facing the center of turn. Such a combine will not only be steered more easily to run straight but also will contribute to a decreased radius of turn. In this case, the combine tends to run at a constant speed whether it advances straight or turns around the center. Therefore, the driver will have to slow down the combine when it must make a "spin" of a very small radius of turn. Thus, the driver have to unwillingly carry out both the operations for turning the combine and for speed change thereof.
A link motion mechanism may be employed in addition to the oil-hydraulic (hereinafter referred to simply as `hydraulic`) steerage so as to automatically slow down the combine when said device is operated, and also to automatically accelerate the combine to its normal speed in harmony with a following operation for causing it to restore its straight-running mode. This way of diminishing the described problems will however give rise to another problem, and indeed, when the combine is adjusted as to its running course relative to a row of grain stalks, its running speed will increase or decrease unintentionally and at random even during normal harvesting works. In such an event, the driver will suffer from a kind of discrepancy between his feeling in driving the combine and its actual motions, disabling him to smoothly and adequately steer the combine.
In general, the driver's manual force applied to a steering wheel or the like parts is transmitted to a `steering gear` or steerage through a train of reduction gears. The output from the reduction gears for turning a mobile agricultural machine for example the combine is proportional to the angular displacement of the steering wheel. If in such a prior art system a particular farm on which the combine is running along a row of grain stalks to be harvested is more slippery for the crawlers than normal farms, then the combine is likely to make a smaller turning action than expected. If on the contrary the particular farm is less slippery than the normal farms, then the combine will make an overaction when turning. As a result, it is remarkably difficult for the driver to steer the combine along the row of grain stalks that have to be reaped and threshed. He will have to make his best efforts to recover a correct turning motion of the combine, relying solely on his driving sense and ability, lest the combine should turn so late or meander so seriously that it would deviate from said row of grain stalks.